This invention relates to a compression release brake system for an internal combustion engine and a method of operating the same to achieve desired noise emission levels.
Engine compression release brakes are well known for providing retarding of vehicles without activation of the vehicle""s service brakes. Examples of known engine compression release brakes are shown in U.S. Pat. Nos. 5,012,778 to Pitzi and 4,741,307 to Meneeley. In general, traditional engine compression release brakes provide retarding by absorbing energy as a result of compressing intake air in the engine""s combustion chamber. The engine""s exhaust valves are opened near the end of the normal compression stroke, thereby preventing energy from being imputed back into the drive train. When the exhaust valves are opened, the pressure in the engine cylinder is released or xe2x80x9cblown downxe2x80x9d, which produces a high level of noise emissions through the engine exhaust system.
The aforementioned compression release brake systems are routinely used on over-the-road or on-highway vehicles, such as delivery truck and semi-tractors that regularly operate in both rural and urban regions. Many jurisdictions have instituted noise level restrictions, especially in residential areas, and traditional compression release brake systems typically produce noise levels that exceed the maximum noise levels permitted by law in many geographic regions. Consequently, vehicle operators are routinely prohibited from operating compression release brakes when operating in noise restricted regions. As a result, the operator must utilize the vehicle""s services brakes to retard or slow the vehicle in cases where a compression release brake could be advantageously used to avoid wear on the service brakes.
Traditional engine compression release brakes, such as those commercially available from Jacobs Manufacturing Company for example, are able to modulate the applied retarding force by selectively operating brake cycles on less than all of the engine cylinders. For example, in a six cylinder engine, brake systems are typically installed such that one portion of the brake system controls braking on one cylinder, another portion of the brake system controls braking on two cylinder together, and a third portion of the system controls braking on the remaining three cylinders. As a result, the vehicle operator can select among six discrete levels of braking by activating one to six of the cylinders. However, such modulation of the brake systems does not significantly alter the noise emission level produced by brake operation, but instead only changes the frequency of noise emissions and/or the cadence the noise emissions. This is due to the fact that resulting noise emissions correspond to the cylinder pressure at the time of pressure release, which is in turn tied to the timing of the pressure release event, which is in turn tied to the fixed shape of the cam that operates a traditional compression release brake. Although de minimis noise reduction may be achieved in traditional systems because lower braking levels produce lower turbo boost and thereby reduce cylinder pressure at the time of release, significantly reduced levels of noise emissions are not achievable in traditional systems even when operating at lower levels of braking or retarding.
Another attempt to reduce noise is illustrated in U.S. Pat. No. 5,357,926 to Hu. In this patent, noise is reduced when the vehicle operator electrically adjusts the xe2x80x9clashxe2x80x9d of the engine brake. xe2x80x9cLashxe2x80x9d is the xe2x80x9cat restxe2x80x9d clearance between the engine brake slave piston and the engine exhaust valve mechanism operated on by the slave piston to produce braking. By reducing the xe2x80x9clashxe2x80x9d, the timing of the braking event is advanced slightly, thereby reducing the cylinder pressure at xe2x80x9cblow down.xe2x80x9d Unfortunately, this approach is not automatic and requires the driver to recognize that he or she is in a noise restricted area and manually change the lash. Additionally, this design only provides one level of adjustment, even though jurisdiction may have varying degrees of noise restrictions. This system also increases the number of components in the vehicle and increases cost. Finally, because the xe2x80x9clashxe2x80x9d is manually changed, the braking system is not capable of automatically providing additional braking power in an emergency, when it would otherwise be desirable to xe2x80x9cignorexe2x80x9d noise restrictions for overriding safety concerns.
This invention is directed to overcoming one or more of the problems identified above.
The present invention includes a method for operating a vehicle having an engine compression release brake, comprising: operating the engine compression release brake in a first mode producing a first level of noise emissions; determining that the vehicle is operating in a noise restricted geographic region; and in response to the determining step, automatically operating the engine compression release brake in a second mode producing a second level of noise emissions lower than the first level of noise emissions.